A known type of shock absorber for retractable landing gear of an airplane comprises a cylinder and a rod that slides in said cylinder, said rod having a fixed end delimiting a bottom chamber of hydraulic fluid that communicates via a diaphragm with a top chamber of hydraulic fluid adjacent to a chamber containing gas under pressure and formed in the top of the cylinder.
In some situations, when the airplane is at rest on the ground, it is desirable to be able to modify the attitude of the airplane, i.e. the inclination of its longitudinal axis.
One possible approach then consists in attempting to change the length of the front landing gear without altering the main landing gear. If the front landing gear can be lengthened, then the desired attitude of the airplane can be obtained while it is stationary on the ground.
Under such circumstances, it is advantageous to provide front landing gear that is adapted to be extensible.
Nevertheless, it is important to avoid confusing the system used for extending the landing gear when the airplane is stationary on the ground, with the means for extending the landing gear so as to enable it to run over irregular ground and even overcome obstacles of considerable size. In the latter case, it is desirable to change the "isothermal" response curve of the shock absorber (variations in shock absorber force as a function of compression stroke), e.g. by providing a structure enabling the shock absorber to have a single chamber on landing but two chambers while running on the ground (after it has been extended), as described in document FR-A-2 601 097.
The shock absorber described in that document thus comprises a moving disk defining the top of a high pressure gas chamber whose bottom is defined by a piston whose rod passes through the moving disk, and a bottom hydraulic fluid chamber which is defined by said piston and the end of the sliding rod, and which is fed from a controllable source connected to the hydraulic supply of the airplane. The structure of that shock absorber is a result of the desired objective, namely passing over bumps while running on the ground, and such a shock absorber is not suitable merely for static extension of the landing gear when the airplane is stationary on the ground.
The static approach to extension for an airplane that is stationary on the ground consists in generating a force equivalent to the static load on the landing gear so as to raise the cylinder of the shock absorber relative to its sliding rod (which rod is in fact stationary, such that extending the shock absorber by causing its sliding rod to move out from the cylinder is, in fact, raising the cylinder of said shock absorber).
An object of the invention is to solve this technical problem by designing a shock absorber whose structure makes it easy to raise the landing gear when the airplane is stationary on the ground, without requiring the airplane's hydraulic generator to be used, i.e. without requiring its engines to be in operation.
Another object of the invention is to provide a shock absorber that is simple in design, for which it is easy to control extension without any risk of disturbances in or leaks from the hydraulic generator circuits of the airplane.